Scorching heat to torrential rain and thunderstorms

  • Observed rainfall in the 24 hours to Wednesday morning, showing the exceptionally high totals in some western areas

    Hail was reported in thunderstorms in Reading on Thursday evening. Credit: Paul Knightley.

    Observed rainfall in just 12 hours on Thursday night, showing the cluster of thunderstorms over central southern and south-east England.

  • Scorching heat to torrential rain and thunderstorms
    17.09.2016 13:19

    As we are now firmly into the middle of September and heading ever-closer to the autumn equinox, one might have expected the weather of the past few days to have been notably cooler and distinctly more ‘autumnal’ compared with the summer just gone. However, the weather of last week has been much more typical of high summer, with temperatures into the 30s Celsius, followed by a thundery breakdown with torrential rain and flash flooding.

    A complex synoptic setup developed on Monday 12th, leading to an exceptional few days of weather. A secondary area of low pressure developed from a much larger ‘parent low’ to the north-west of the British Isles. This became rooted over the Bay of Biscay, just to the west of France. At the same time, high pressure was centred across Scandinavia and the Baltic Sea, blocking the advance of any Atlantic influence. This had the effect of drawing up a hot and humid flow of air from the south-east, forcing temperatures across most parts of the UK several degrees above the seasonal average maximum, which for mid-September is around 19°C in London.

    The heat as most evident in East Anglia and south-east England where, for three consecutive days (Tuesday-Thursday) temperatures exceeded 30°C.
    On Tuesday, 13th, the UK recorded its hottest day of 2016 when the thermometer at Gravesend soared to 34.4°C. This surpasses the previous hottest day of the year by half a degree, with the same location climbing to 33.9°C on 24th August. Amongst the meteorological community, there have been some doubts cast over the reliability of the site at Gravesend, due to its sheltered location in a southerly flow. Nevertheless, back on 24th August, Cavendish in Suffolk also peaked at 33.8°C.

    While the eastern half of the country basked in hot sunshine on Tuesday, a frontal boundary aligned to the west gave sufficient instability for severe thunderstorms to break out. The storms were relentless in parts of south-west England, with Culdrose in Cornwall receiving 48mm of rainfall in 24 hours. A cluster of thunderstorms moving northwards from the West Midlands and into Greater Manchester on Tuesday evening led to flash flooding and the cancellation of Manchester City’s Champions League match against Borussia Mönchengladbach.

    By Thursday, 15th, it was across central southern and south-eastern parts of England where the heat and humidity began to break down as severe thunderstorms developed. Starting across north-west London during the afternoon, what began as a very isolated thunderstorm soon turned into a mass of heavy and thundery rain during the evening and overnight.
    Hourly rainfall totals were testament to how intense and localised the thunderstorms really were. During Thursday night, 38mm of rain fell at Swanage in Dorset, but most notably, 37.8mm of this fell in just one hour. Into Friday daytime and the storms continued across East Anglia, with 35mm recorded in one hour at Santon Downham in Suffolk. By the end of the day, more than two inches of rain had fallen there with 57mm recorded in the 12 hours to 1900BST.

    There is only a handful of years in which the hottest day occurred outside of meteorological summer (June - August). Looking back on the archives, it is often during years with disappointingly cool summers that the hottest day occurs unusually early or late. For instance, the summer of 1962 was one of the few years where temperatures failed to reach 30°C. The hottest day of that particular year was recorded on 3rd September, when Writtle in Essex climbed to 27.8°C. This is not the case with 2016, as the summer just gone has seen several hot spells, one of which saw the temperature climb to 33.5°C in Brize Norton, Oxfordshire, back on 19th July. That same day, Jersey Airport on the Channel Islands (outside of the UK) reached 35.2°C. As mentioned above, late August also saw a hot spell with temperatures surpassing 30°C in several locations once again.

    After what has been a remarkable week of weather, the coming few days will see a return to more seasonal conditions with temperatures close to what we’d expect from mid-September. This is thanks to a more zonal flow, whereby areas of low pressure are able to track from west to east from the Altantic and onto our shores, bringing occasional spells of wet and breezy weather, although certainly not on the scale of the very localised torrential rain of recent days.

    By: John Lee